1. Field of the Invention
This invention relates to recording and reproducing video and audio signals and, more particularly, to such apparatus wherein the audio signal that is recorded and reproduced with the video signal is subjected automatically to different types of signal processing, depending upon an indication of the type of audio signal being recorded.
2. Description of the Prior Art
Conventional video tape recorders record a color video signal by separating the luminance and chrominance signals and then shifting the frequency domains of the separated signals. Typically, the luminance signal is frequency modulated to a relatively high frequency band and the chrominance signal is frequency converted to a relatively low frequency band. The lower chrominance band and upper luminance band are mixed to form a processed video signal which is recorded in successive oblique tracks on a magnetic tape by rotary magnetic heads. An audio signal also is recorded on the same tape as the processed video signals; and in one type of video tape recorder, the audio signal is recorded in one or more longitudinal tracks adjacent an edge of the video tape.
To increase the recording density of the processed video signal, another type of video tape recorder transports the magnetic tape at a relatively slow speed. While this improves the recording density and permits a significantly longer recording time, slow tape speeds tend to deteriorate the quality of the audio signal. Accordingly, in this type of video tape recorder, the audio signal is frequency modulated and mixed with the processed video signal to be recorded by the rotary heads in the same oblique tracks as the video signal.
A typical frequency spectrum representing the mixed audio and processed video signals is illustrated in FIG. 1, wherein the abscissa represents frequency and the ordinate represents signal level. It is seen that the frequency-converted chrominance signal C.sub.c is recorded with a carrier frequency f.sub.c in a range that is well below the frequency band occupied by the frequency modulated luminance component Y.sub.f. The audio signals are frequency modulated and occupy a range disposed within the relatively narrow space between the frequency-converted chrominance band and frequency modulated luminance band. In a typical video tape recorder of desired quality, the audio signals comprise stereophonic signals; and the two channels are recorded in two separate bands A.sub.1 and A.sub.2. Thus, one channel of the stereophonic signals frequency modulates one carrier frequency, for example, f.sub.a1 and the other channel of stereophonic signals modulates another carrier frequency, for example, f.sub.a2. Conventionally, the stereophonic channels are formed as a sum channel wherein the left-channel and right-channel audio components are summed (L+R), and a difference channel wherein the audio components are subtracted (L-R). As an example, the summed components (L-R) modulate carrier frequency f.sub.a1 and the difference components (L-R) modulate carrier frequency f.sub.a2.
In one application of recording stereophonic signals and video signals on a video recorder, carrier frequency f.sub.a1 is on the order of about 1.5 MHz and is frequency modulated to exhibit a frequency deviation on the order of about 100 to 150 KHz. Carrier frequency f.sub.a2 is on the order of about 1.7 MHz and is frequency modulated to exhibit a frequency deviation which also is on the order of about 100 to 150 KHz. The carrier frequency of the upper sideband of the frequency modulated luminance signal Y.sub.f ranges from a low frequency of about 4.2 MHz to a high frequency of about 5.4 MHz. The lower frequency (4.2 MHz) of the frequency modulated luminance signal represents the so-called sync tip, that is, the magnitude of the horizontal synchronizing signal, and the upper frequency (5.4 MHz) of the frequency modulated luminance signal represents the white peak level, that is, the maximum amplitude of the luminance signal. Thus, the carrier frequency of the frequency modulated luminance signal varies between the sync tip frequency f.sub.s =4.2 MHz and the white peak frequency f.sub.p =5.4 MHz. Finally, the carrier frequency f.sub.c of the frequency converted chrominance signal C.sub.c typically exhibits a color subcarrier frequency on the order of about 743 KHz.
As illustrated in the frequency spectrum of FIG. 1, the level of the frequency modulated luminance signal is greater than the level of the frequency-converted chrominance signal which, in turn, is greater than the level of the frequency-modulated audio signal components A.sub.1 and A.sub.2.
When the video tape recorder is used to record stereophonic audio signals with the processed video signals, the recording circuitry typically is provided with a stereophonic matrix circuit for producing the sum signal (L+R) and the difference signal (L-R) from the separate left-channel and right-channel audio signals supplied thereto. The reproducing circuitry of such a video tape recorder typically includes a frequency demodulator to demodulate audio signal components A.sub.1 and A.sub.2 and thereby recover the sum signal (L+R) and difference signal (L-R), respectively, and these recovered sum and difference signals are applied to a receiver matrix circuit to reproduce therefrom the left-channel and right-channel signals L and R. There are times, however, when it is desired to record two separate audio channels on the magnetic tape that are not stereophonic signals. For example, audio component A.sub.1 may comprise a main audio channel and audio component A.sub.2 may comprise an auxiliary audio channel. A typical application of main and auxiliary audio channels is found in bilingual audio processing wherein the main channel contains information of a primary language and the auxiliary channel contains information of a secondary language. For example, in recording a video program, the main channel may represent the language in which the original actors speak (such as a foreign language) and the auxiliary channel may represent a dubbed, translated language (such as an English translation). In this environment, when the main and auxiliary audio channels are played back, a user may select one or the other for sound reproduction.
Video tape recorders having the capability of recording either stereophonic audio signals or main/auxiliary channel audio signals should be provided with means to supply the carrier frequencies f.sub.a1 and f.sub.a2 either with sum and difference stereophonic signals or with main and auxiliary audio channels, and means should be provided to selectively enable or disable the stereophonic matrix circuit. Likewise, the reproducing circuitry should be provided with means to recover the main/auxiliary audio channels or the sum and difference stereophonic signals, depending upon which type of audio signal is recorded (i.e. whether bilingual or stereophonic audio signals are recorded). Here too, the reproducing circuitry should include means to selectively enable and disable the reproducing matrix circuit. Stated more generally, when stereophonic audio signals are to be recorded and reproduced, such signals are subjected to one type of audio processing; but when bilingual (or other types of) audio signals are to be recorded and reproduced, those bilingual (or other types of) audio signals are subjected to a different type of audio processing. Preferably, if the recorder is to have the capability of recording various different types of audio signals, each subjected to a respectively different type of audio processing, the video recorder should include the requisite audio processors and means to select or match the proper processor with the type of audio signal being recorded. Of course, for compatibility, reproducing circuitry likewise should include different types of audio processors with means to match the proper processor with the particular type of audio signal being reproduced. Such selecting and matching of the proper audio processor to the particular type of audio signal being recorded/reproduced may be effected by manually operated switches wherein a user operates a particular switch to select a particular audio processor compatible with the type of audio signal being recorded. Likewise, during reproduction, the user carries out a similar manual switching operation.
While the use of manually operated switches in the recording circuitry presents no difficulty because the user is acutely aware of the type of audio signal being recorded, a user may not be aware of the proper switch to operate during reproduction because he may not be aware of the type of audio signal that had been recorded. Still further, automatic switching likewise may be difficult to implement. For example, although a pilot signal normally is contained in a stereophonic audio signal and, thus, the pilot signal may be detected and used to select a stereophonic matrix circuit during recording, the pilot signal may not be readily available to control a similar automatic switching of the proper audio processing circuit in the reproducing circuitry.